1. Technical Field
The present invention relates to a backlight module circuit of a liquid crystal display device, and more particularly to a backlight module protection circuit of a liquid crystal display device.
2. Description of the Related Art
In recent years, cold cathode fluorescent lamps (CCFLs), functioning as light sources of backlight modules, are commonly applied to TFT-LCD devices. For increasing the image quality and display effect of the TFT-LCD devices, how to dynamically adjust light source luminance of the CCFLs is a main research scope in the related TFT-LCD display field.
Moreover, in order to meet the requirements of energy savings, high dynamic contrast ratio (DCR) and dynamic display, the light source of the backlight module is expected to perform a real-time luminance adjustment to achieve higher image display quality. In this situation, a current flowing through the cold cathode fluorescent lamps is dynamically changed. Because of the real-time luminance adjustment, when an abnormal situation occurs on the cold cathode fluorescent lamps such as being collided or fallen off, which would result in the cold cathode fluorescent lamps being open circuit. If a protection circuit of the backlight module could not effectively and rightly perform protection function, which would endanger personnel safety and cause operation error of the liquid crystal display device.
Of course, the luminance adjustment of the backlight module may be manually performed by an user in response to an operation environment where the user located in, or automatically performed triggered by a light sensor in the liquid crystal display device sensing the operation environmental luminance.
FIG. 1 is a schematic block diagram of a conventional backlight module using cold cathode fluorescent lamps as a light source thereof. The backlight module 10 includes a control circuit 12, a driving circuit 14, a lamp feedback circuit 16, a protection circuit 18, and a fluorescent lamp set 20 having multiple fluorescent lamps 202. The fluorescent lamp set 20 acts as the light source of the backlight module 10. The protection circuit 18 is comprised of a comparator 182.
Since the conventional protection circuit 18 uses a protection command signal V-command with a fixed value, when the fluorescent lamp set 20 produces a low luminance and further is abnormal or even open circuit, the protection circuit 18 might not be able to detect the abnormal situation in real-time, resulting in the occurrence of issues such as endangering personnel safety and operation error of the liquid crystal display device. Operation principles of signals in the backlight module 10 will be described below in detail.
FIG. 2A is a schematic view of a feedback signal V-fb and the protection command signal V-command when the fluorescent lamp set 20 is abnormal in all-ON state. As illustrated in FIG. 2A, an analog dimming signal A-dim is set at a maximum value, or a duty cycle of a digital dimming signal is set at 100%, so that an amplitude of a lamp current Ilamp is adjusted to be at a maximum value and the fluorescent lamp set 20 is maintained at the all-ON state.
It is clear that, when the fluorescent lamp set 20 is abnormal or even a lamp(s) thereof is/are open circuit, since a momentary load of the fluorescent lamp set 20 is decreased, which causes the lamp voltage Vlamp is increased correspondingly. In this moment, after the lamp feedback circuit 16 obtains the lamp voltage Vlamp and then converted the obtained lamp voltage Vlamp into the feedback signal V-fb, the feedback signal V-fb would be higher than the protection command signal V-command. Accordingly, the protection circuit 18 outputs a low level of comparing result signal Vp, to enable the control circuit 12 to turn off the fluorescent lamp set 20 and thereby achieving the purpose of protecting the fluorescent lamp set 20.
FIG. 2B is a schematic view of the feedback signal V-fb and the protection command signal V-command when the fluorescent lamp set 20 is abnormal during analog dimming. As illustrated in FIG. 2B, the purpose of decreasing the lamp voltage Vlamp and the lamp current Ilamp can be achieved by decreasing the analog dimming signal A-dim.
It is clear that, during the analog dimming, the lamp voltage Vlamp is relatively low, in this situation, even if the fluorescent lamp set 20 is abnormal or even a lamp(s) thereof is/are open circuit, an increase of amplitude for the lamp voltage Vlamp is limited. After the lamp feedback circuit 16 obtains the lamp voltage Vlamp and then converted the obtained lamp voltage Vlamp into the feedback signal V-fb, the feedback signal V-fb may be still lower than the protection command signal V-command. Accordingly, the protection circuit 18 outputs a high level of comparing result signal Vp, to enable the control circuit 12 still to normally work and thereby resulting in the damage of the fluorescent lamp set 20.
FIG. 2C is a schematic view of the feedback signal V-fb and the protection command signal V-command when the fluorescent lamp set 20 is abnormal during digital dimming. As illustrated in FIG. 2C, the purpose of decreasing a root-mean-square value of the lamp current Ilamp can be achieved by decreasing a digital dimming signal V-dim.
Likewise, during the digital dimming, an effective value of the lamp voltage Vlamp is relatively low, in this situation, even if the fluorescent lamp set 20 is abnormal or even a lamp(s) thereof is/are open circuit, an increase of amplitude for the lamp voltage Vlamp is limited. After the lamp feedback circuit 16 obtains the lamp voltage Vlamp and then converted the obtained lamp voltage Vlamp into the feedback signal V-fb, the feedback signal V-fb may be still lower than the protection command signal V-command. Accordingly, the protection circuit 18 outputs a high level of comparing result signal Vp, to enable the control circuit 12 still to normally work and thereby resulting in the damage of the fluorescent lamp set 20.
In summary, since the protection circuit 18 of the conventional backlight module 10 uses the protection command voltage V-command with the fixed value, once it is needed to achieve higher image display quality by decreasing the root-mean-square value of the lamp current Ilamp using the analog dimming or digital dimming, the lamp voltage Vlamp and the feedback signal V-fb would be decreased correspondingly. Therefore, once the fluorescent lamp set 20 becomes abnormal, the feedback signal V-fb may be still lower than the protection command signal V-command, so that the protection circuit 18 would wrongly judge that the multiple fluorescent lamps 202 still are normally working and thereby outputs the high level of comparing result signal Vp, which could not activate the protection mechanism of the backlight module 10.